1. Field of the Invention
This invention relates generally to rising stem gate and globe valves and actuators, and, more particularly, to packingless hermetic rising stem gate and globe valves which have a harmonic drive.
2. Background Art
Conventional gate valves are typically classified as either “rising stem” or “non-rising stem” valves. On the a “non-rising stem” gate valve, the valve stem is threaded on the lower end into the gate and as the handwheel on the stem is rotated, the gate travels up or down the stem on the threads, while the stem remains vertically stationary. In a “rising-stem” gate valve, the stem is attached to the gate at the lower end such that the gate and the stem rise and lower together as the valve is operated. In conventional globe valves, a horizontal disk or a plug is connected at the bottom end of the stem to open or close fluid flow through the valve as the stem is raised and lowered. The present invention is particularly suited for use with “rising-stem” valves, such as gate valves and globe valves.
Conventional gate valves generally comprise a valve body having inlet and outlet conduits with a chamber therebetween and the gate is disposed between a pair of gate seats mounted within the chamber and the upper end of the valve stem is externally threaded and threadedly engaged on an internally threaded portion of the valve bonnet wherein the threaded portions of the stem and bonnet are completely engaged circumferentially and the stem is typically sealed with the bonnet by means of a seal such as an o-ring, compressed sealant, or chevron packing.
When the valve is in its closed position, pressure upstream acts to urge the gate against the contact face of the downstream valve seat with a force proportional to pressure, which increases the frictional forces between the gate and downstream seat, which in turn increases the force necessary to rotate the stem to open the valve. The complete circumferential engagement of the threaded portions of the stem and the bonnet also increases the frictional contact surface area, which in turn increases the force necessary to rotate the stem to open or close the valve.
The stem packing sealing systems described above are subject blow out under high pressure and to wear and leakage over time under normal operating conditions, and thus require periodic replacement or maintenance to maintain the integrity of the seal.
“Harmonic drive” gear trains are known in the art. In one known design, a motor rotates a “wave generator” which is an oval or elliptical shaped member, which flexes diametrically opposite portions of a surrounding flex-spline gear, which is inside an inner gear. As the diametrically opposite teeth of the flex-spline gear contact the teeth of the outer gear, the rotatable one of the gears rotates with respect to the non-rotatable one of the gears.
“Harmonic screw” actuator devices are also known wherein rotary motion of a power input component is converted into linear motion of an output component by means of a strain wave imposed upon a flexible sleeve member, which is flexed into intermittent screw engagement by a member which is typically motor-driven to rotate relative to the periphery of the flexible sleeve. Devices of this general type are typically employed to operate levers, arms, cranks, slides or the like, such as for opening and closing doors, gates, slides, butterfly damper valves, etc.; and wherever push-pull power outputs are required.
Musser, U.S. Pat. No. 2,979,964 discloses a linear actuator with a nut element having an inside thread, a screw element having an outside thread within the nut, one of the nut and the screw elements being relatively rigid radially and the other being relatively flexible radially, the threads on the nut and the screw being of the same hand, of the same thread form and the same pitch, the threads of the nut and the screw engaging at circumferentially spaced locations with intermediate locations of non-engagement, and a wave generator for deflecting the flexible element into engagement with the other element at the plurality of locations and for progressing the locations of engagement of the threads around the circumference. In one embodiment, the wave generator is an external circular member having an ellipsoidal internal surface that surrounds the nut and causes the cross section of the nut and the interior contour of the nut at the threads to be ellipsoidal engaging the screw threads at circumferentially spaced locations whereby rotation of the external wave generator by a spanner wrench causes relative motion axially between the screw and the nut.
Hoos, U.S. Pat. No. 3,868,867 discloses a valve actuator utilizing a differential harmonic drive which includes three or more coaxial wheels. A wave generator wheel carries rotatable planet shafts to orbit them around the other coaxial wheels, a stationary or reference wheel, and one or more rotatable output wheels. Driving engagements between the stationary wheel and its orbiting planet wheels impart rotation to the planet shafts, so that driving planet wheels on the same shafts rotate the output wheels as they orbit around them. The stepped-up drive ratio imparted by the stationary wheel is different from the stepped-down ratio between the output wheel and its planet wheels, and the differential governs the overall ratio of the system. For a variation in overall drive ratio, either of two or more output wheels may be activated selectively or the output wheel and its associated planet wheel may constitute a variable ratio drive system.
Gernerke et al, U.S. Pat. No. 3,979,971 discloses a differential drive in which a reference sprocket is stationary for power drive and rotated for manual drive. In power drive, planet sprockets are orbited around the reference sprocket and rotated thereby. Drive planet sprockets on the same shafts drive an output sprocket at a slightly different ratio from the reference drive as they orbit around it, whereby there is a slow net rotation imparted thereto. In manual drive, the planet shaft axes are fixed and the reference wheel is rotated to drive the planet sprockets and rotate the output sprocket.
Ulbing, U.S. Pat. Nos. 4,557,153 and 4,848,184 discloses improved harmonic screw type actuators wherein rotary motion of the power input component is converted into linear motion of the output component by means of a strain wave imposed upon a flexible sleeve member of the assembly, which is flexed into intermittent screw engagement by a member which is motor-driven to rotate relative to the periphery of the flexible sleeve; thereby causing progressively advancing/retreating inter-engagements of thread-like surfaces formed on the axially stationary and axially movable components of the system.
Pochlau, U.S. Pat. No. 6,772,655 discloses a harmonic drive embodied in a hollow gear. The harmonic drive directly provides a given correcting translatory variable which is defined by the speed of rotation of the output ring and motor driven harmonic generator when the output ring axially displaces a threaded rod which freely extends in a coaxial manner through a longitudinal bore in the harmonic drive and engages with the output ring, the axial displacement occurring as a result of the rotational movement of the inner thread of the output ring, whereby the threaded rod is secured to the housing and prevented from rotating. The threaded rod can move in a linear manner in relation to the gear motor when the gear motor is installed in a stationary manner or when the treaded rod is installed in a stationary manner.
Marsh et al, U.S. Pat. No. 7,017,882 discloses a valve assembly that includes a valve stem which is connected with a valve and a drive member disposed in engagement with the valve stem. A gear assembly which includes a harmonic drive unit disposed in a coaxial relationship with the valve stem is operable to rotate the drive member to effect axial movement of the valve stem. The harmonic drive unit includes a flex spline which extends around a portion of the valve stem, a rigid circular spline which extends around a portion of the valve stem; and a wave generator which effects deflection of at least a portion of the flex spline.
Osle, U.S. Pat. No. 7,412,910 discloses a mechanism for transforming a rotational movement into a linear movement, comprising a non-deformable nut, that is prevented from rotating but not from axially moving, a radially deformable tubular screw, having freedom of rotation but not of axial movement and having a pitch equal to that of the nut but of a slightly external smaller diameter than the internal diameter of the nut. A pushing device inside the screw, with the ability to rotate and to push against the internal wall surface of the screw at least at three circumferentially equidistant points so as to radially deform the screw wall outwards, until bringing the screw wall to rest against and screw into the nut at an equal number of points. The pushing device may be independent rolling elements such as rolling bearings on a core on a support or may be a cam in contact with the screw or through rolling elements.
Baudendistel et al, U.S. Pat. No. 7,527,130 discloses a harmonic drive linear actuator which includes a first annular member defining a longitudinal axis and lying on a plane perpendicular to the longitudinal axis and is relatively flexible along a direction parallel to the plane. A second member is substantially coaxially aligned with the first member to define opposed substantially cylindrical surfaces fixed for non-relative rotation about the longitudinal axis. An actuator is provided for flexing the first annular member into at least two spaced-apart points of contact between the opposed surfaces and for sequentially flexing the first member to rotate the at least two points of contact circumferentially about the axis. The first and second surfaces define cooperating thread-forms thereon, which selectively engage to effect controlled, bidirectional relative axial displacement between the members in response to sequential flexure of the first member. The linear actuator can be conjoined with an actuator piston of a vehicle brake caliper assembly.